Biogeochemistry of the Ballarat East goldfield, Victoria, Australia

Abstract

Samples of outer bark, twigs and leaves were collected from Pinus radiata and three varieties of native trees and shrubs ( Acacia melanoxylon, Cassinia aculeata, Eucalyptus) in an area of pine plantation and remnant native forest across the Ballarat East goldfield. The results of this study indicate significant enrichment of Au, As, Ce, Cr, Sm, Sc and Th in both exotic and native plant tissues associated with areas of historical gold mining activity and hydrothermal alteration, as well as their probable extensions. Gold in the bark of 28-year old P. radiata provides the best anomaly definition, whereas As in the same material is less reliable. A suite of REE and other high field strength elements from the bark samples, including Ce, La, Sm, Sc and Th, also correlate well with the inferred position of mineralized trends, as well as with Fe. In contrast, Au and As in dried needle litter failed to correlate with mineralization. Trace elements in five- to ten-year old P. radiata twigs and needles also reveal the position of mineralized trends, but the anomaly to background ratio is less than that observed in the older bark samples. A more favourable response was obtained for the same traverse using Au, As, Ce, La, Sm, Sc and Th data from C. aculeata twigs and leaves. The latter is a widely distributed shrub in central Victoria, particularly in areas of forest regrowth, and thus has some potential for biogeochemical prospecting purposes. Blackwood trees ( Acacia melanoxylon) were sparsely distributed in the study area, and samples of twigs and leaves show very low levels of Au and As, indicating that this is not a suitable plant for biogeochemical surveys in the area without enhancement by ashing. Gold, As, Ce, Cr, La, and Sm, from bark, leaves and twigs from three varieties of Eucalyptus also show a favourable response to the southern extensions of the Indicator and Promised Land Lines, regardless of species. However, elevated Au values of up to 147 ppb in some unashed Eucalyptus bark samples are probably due to contamination by airborne dust generated by nearby anthropogenic activities. Gold in these samples correlates poorly with other indicator elements, whereas Fe shows strong positive correlations with As, Ce, Cs, Cr, La, Sc, Sb, Th, Zn and Ba. Decoupling of Au from other elements in dust may be characteristic of contamination, and indicates that biogeochemical data from rough barked Eucalyptus species should be treated with caution in areas of high dust levels.